The present invention relates to fuel cells and, more particularly, to a fuel cell separator plate in which the reactant and cooling flow fields are integrated into a single plate. An optional segmented gas diffusion medium may permit the reactant and cooling functions of the cell to be effectively segregated, and also may permit direct hydration of the membrane of the fuel cell.
High current density (and therefore high power density) operation of fuel cell stacks produces a large amount of heat. It is necessary to remove this generated heat in order to operate efficiently. Historically, this has been accomplished by circulating cooling fluid through a plurality of cooling plates that are interspersed evenly throughout the fuel cell stack. These plates are dedicated solely to the cooling function.
A major drawback of employing these plates is the added complexity to the system, and an increase in the number of stack components, resulting in an increase of the cost of fabrication.
Additionally, efficient operation at high current densities and/or operation at high temperatures (critical for CO-tolerance), may require that the water content of the polymer electrolyte membrane be maintained. Historically, this has been achieved by pre-humidifying one or both of the fuel cell reactant gases so that the un-reacted gases do not dry out the membrane as they leave the stack.
Unfortunately, such pre-humidification in practice adds a layer of engineering complexity, cost, and sometimes even additional parasitic power consumption. These drawbacks reduce the attractiveness of the fuel cell system as an alternative power source.
The present invention seeks to provide an improved fuel cell that eliminates all of the aforementioned problems.
The fuel cell of this invention features a new type of design that permits integration of cooling the fuel cell and reactant flow functions into a single, separating plate element.
As aforementioned, power efficiency and high temperature operability are both dependent on maintaining the water content of the electrolyte membrane. The segmented gas diffusion medium (GDM) in one embodiment of this invention, however, permits direct membrane hydration, getting the water or water solution directly to the electrolyte membrane in the gaps between the GDM segments, but simultaneously keeping it away from where it is not wanted. Thus, the water is prevented from interfering with the reactant flow field and the GDM.
In accordance with the present invention, there is provided a modified separator plate and associated manifolding. The separator plate integrates the reactant flow function and cooling flow function of a fuel cell into a single separating element. An additional embodiment features a segmented GDM that is constructed to provide direct hydration to the polymer electrolyte membrane. This is achieved by one of two methods. The first method consists of putting the water or water solution coolant flow field in direct contact with the immediately adjacent membrane, by providing a gap in the gasket disposed between the coolant flow field and the electrolyte membrane.
An alternative of the above approach provides the second method. The membrane hydration can be accomplished by use of a microporous wicking material disposed between the membrane and the coolant flow field of the separator plate. The wick, while being microporous enough to serve as an effective obstacle to bulk water flow, has water transport properties that are superior to those of the membrane. This results in an increase in the rate of water transfer to areas of the membrane that are electrochemically active. Both of the aforementioned hydration methods accomplish the hydration of the electrolytic membrane without exposing liquid water to the GDM or to the reactant gas flow fields.
It is an object of this invention to provide an improved fuel cell.
It is a further object of this invention to provide a fuel cell that integrates the cooling and reactant flow functions into a single plate.
It is another object of this invention to provide an unsegmented GDM and modified separator plate with provisions for predominantly containing cooling water within its flow field in the adjacent modified separator plate.
It is another object of the invention to provide a fuel cell having a modified separator plate and an optional, segmented GDM for use in integrating the cooling and reactant flow functions into a single separator plate.
It is yet a further object of this invention to provide a segmented GDM and modified separator plate for a fuel cell stack that provides direct hydration to the electrolytic membrane.